The present invention relates to an optical element having a reflection suppressing function (anti-reflection function).
Optical elements such as lenses used in optical systems are manufactured by using a transparent base material such as optical glass or optical plastic. Such a transparent base material has a large refractive index, so that a reflectance thereof is high. A high reflectance of the transparent base material reduces an amount of effective light reaching an image surface, and generates unnecessary reflection which causes ghost or flare. Therefore, it is necessary to provide an anti-reflection function to the optical element manufactured by using the transparent base material.
To provide the anti-reflection function to the optical element, an anti-reflection film is generally formed on a surface of the optical element (transparent base material). The anti-reflection film is formed by laminating thin film layers on the surface of the transparent base material, based on a general optical interference theory.
Methods for forming anti-reflection films include a dry method (vacuum film forming method) and a wet method (wet film forming method). The dry method coats a surface of the transparent base material with metal fluoride or metal oxide having a low refractive index, as with a vapor deposition method and a sputtering method. Further, the wet method applies coating liquid containing a low refractive index material to the surface of the transparent base material by a dipping method, a spin coat method or the like, and then dries or fires it.
A thickness (hereinafter also referred to “film thickness”) of the anti-reflection film is frequently designed such that an anti-reflection effect becomes largest at a central part of the optical element (hereinafter referred to as lens), that is, at a portion where an incident angle of a light ray is 0 degree. The anti-reflection film designed as above has a uniform film thickness over the entire lens surface.
However, when a light ray vertically enters the central part of the lens on which the anti-reflection film is formed and light rays enter a peripheral part of the lens in parallel with the above light ray, incident angles of the light rays become large at the peripheral part. Therefore, anti-reflection performance at the peripheral part becomes lower than that of the central part of the lens.
Japanese Patent Laid-open No. 2004-333908 discloses an anti-reflection film having an optical film thickness which makes its reflectance lowest for an entering/emerging light ray at arbitrary positions on a lens surface. According to a general optical interference theory, an optical path length difference between reflected light at a surface of the anti-reflection film and reflected light at a boundary surface between the anti-reflection film and the transparent base material is an odd-numbered times of one-half of a wavelength of the light. Accordingly, these reflected lights interfere with each other to be mutually weakened. Japanese Patent Laid-open No. 2004-333908 utilizes this theory. According to the theory, it is necessary to increase the thickness of the anti-reflection film from the central part of the lens toward the peripheral part thereof.
However, the anti-reflection effect of the anti-reflection film manufactured based on the general optical interference theory depends on the thickness thereof, so that a difference between an actual film thickness and a designed film thickness makes it impossible to obtain a satisfactory anti-reflection effect. Therefore, a highly accurate film thickness control is required for forming the anti-reflection film.
Further, another method for providing the anti-reflection function to the optical element forms a structure having a concave-convex shape finer than a wavelength of entering light (hereinafter referred to as “use wavelength”) on the surface of the transparent base material.
In the concave-convex structure finer than the use wavelength, the entering light behaves as if the structure is a uniform medium since the entering light cannot recognize the concave-convex shape. The concave-convex structure has a refractive index according to a volume ratio of a material forming the concave-convex shape, thereby showing a low refractive index that cannot be obtained by normal materials. Consequently, the use of such a concave-convex structure can achieve a higher anti-reflection performance than that of anti-reflection films made of low refractive index materials.
Methods for forming the above described concave-convex structure include a method applying a film in which fine particles having a particle diameter smaller than a use wavelength are dispersed onto a surface of a transparent base material (refer to Japanese Patent No. 3135944), and a method forming a periodic concave-convex structure by pattern formation by using a microfabrication apparatus (refer to Japanese Patent Laid-open No. 50-70040). Further, the methods include a method forming a concave-convex structure of petal-shaped alumina by using a sol-gel method (refer to Japanese Patent Laid-open No. 09-202649).
However, formation of such a concave-convex structure requires complicated processes. Further, the concave-convex structure is formed by using some limited materials, which reduces a degree of freedom in design of the refractive index. Therefore, there is a problem that high anti-reflection performance of the concave-convex structure can be obtained only when using transparent base materials having limited refractive indexes.
Japanese Patent Laid-open No. 2005-275372 discloses a method providing, between a concave-convex structure and a transparent base material, a thin film layer (intermediate layer) formed of a material having a refractive index between those of a material forming the concave-convex structure and the transparent base material. This disclosed method changes the refractive index gradually from the concave-convex structure to the transparent base material, which can reduce reflection at the boundary surface of the transparent base material. Further, selection of a material forming the thin film layer can provide a lot of options in selecting the transparent base material.
The dry method described above such as the sputtering method and the vapor deposition method arranges a vapor deposition source such that it faces the central part of the lens to form a thin film layer. This arrangement enables formation of an anti-reflection film whose anti-reflection performance becomes the maximum at the central part of the lens as designed.
However, when anti-reflection film formation is performed by the dry method on a lens surface having a small curvature radius, an incident angle of a vapor deposition material increases toward the peripheral part, thus reducing the film thickness toward the peripheral part. Generally, when a film thickness at an incident angle of 0 degree is defined as D, a film thickness at an incident angle of 60 degrees is about D×cos(60°), which is about half of the film thickness at the incident angle of 0 degree. Consequently, it is difficult to make the film thickness at the peripheral part larger than that at the central part of the lens based on the optical interference theory by using the dry method. A method disposing a mask can make the film thickness at the peripheral part than that at the central part of the lens, which, however, requires huge equipment.
On the other hand, the wet method described above such as the dipping method and the spin coat method has a low controllability of the film thickness, which makes it difficult to achieve a highly accurate film thickness control.
Further, the anti-reflection structure disclosed in Japanese Patent Laid-open No. 2005-275372 in which the thin film layer is disposed between the concave-convex structure and the transparent base material has better antireflection characteristics in a large wavelength band in comparison with the anti-reflection structure constituted only by the anti-reflection film formed based on the interference theory or the concave-convex structure. However, in the disclosed anti-reflection structure, small reflection occurring at the peripheral part of the lens surface having a small curvature radius generates ghost and flare. Thus, further development in anti-reflection performance is desired.